Plasma arc torches are widely used for workpiece processing, e.g., the cutting, piercing, and/or marking of metallic materials (i.e., elemental metals, metal alloys. etc.). A plasma arc torch generally includes an electrode mounted within a body of the torch (i.e., a torch body), a nozzle having a plasma exit portion (sometimes called an exit orifice or exit port) also mounted within the torch body, electrical connections, fluid passageways for cooling fluids, shielding fluids, and arc control fluids, a swirl ring to control fluid flow patterns in a plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum (i.e., an ionized plasma gas flow stream). Gases used in the plasma arc torch can be non-oxidizing (e.g., argon, nitrogen) or oxidizing (e.g., oxygen, air).
In operation, a pilot arc is first generated between the electrode (i.e., cathode) and the nozzle (i.e., anode). Generation of the pilot arc may be by means of a high frequency, high voltage signal coupled to a DC power supply and the plasma arc torch, or any of a variety of contact starting methods. In some configurations, a shield is mounted to the torch body to prevent metal that is spattered from the workpiece (sometimes referred to as slag) during processing from accumulating on torch parts (i.e., the nozzle or the electrode). Generally, the shield includes a shield exit portion (also called a shield orifice) that permits the plasma jet to pass therethrough. The shield can be mounted co-axially with respect to the nozzle such that the plasma exit portion is aligned with the shield exit portion.
Arc stability has been a limitation on plasma arc cutting systems. In particular, maximum severance thickness, cutting speed, and arc stability through corners and crossing kerfs present can negatively affect the quality of workpiece processing, for example, by creating an uneven (i.e., rough) processed surface. A known method of improving arc stability can be achieved by coaxial shield flow designs, for example, as described in U.S. Pat. No. 6,207,923 to Lindsay (and assigned to Hypertherm, Inc. of Hanover, N.H.). Coaxial shield flow designs generally use an axially directed column of shield gas to surround a plasma jet. The column of shield gas can be created by an extended nozzle tip disposed within a funnel-shaped shield.
FIGS. 1A-1B depict a configuration that can provide a column of shield gas using coaxial shield flow designs. FIG. 1A shows a torch 100 including an electrode 105 mounted in a spaced relation with a nozzle 110 to form a plasma chamber 115 therebetween along the longitudinal axis A. A shield 120 is mounted relative to the nozzle 110 such that a gas passageway 125 is formed therebetween. The nozzle 110 defines a plasma exit portion 130 through which the ionized plasma jet (not shown) is ejected from the torch. The shield 120 defines a shield exit portion 135 that is substantially coaxial with the plasma exit portion 130. A secondary gas (not shown) flows through the gas passageway 125 and exits the torch 100 via the shield exit portion 135 to form a column of secondary gas about the plasma jet. The shield 120 also defines a plurality of exit holes 140. Some secondary gas passes through the holes 140 during operation of the torch 100. This secondary gas prevents spatter from the workpiece from accumulating on the shield 120 or nozzle 110 and/or moves molten metal away from the plasma jet.
Nozzles 110 and shields 120 of this design show marked improvement in cutting speeds, arc stability, and piercing capacity over previous configurations. However, coaxial shield flow designs have failed in water-cooled plasma arc torch systems operating over approximately 260 Amps and in air-cooled plasma arc torch systems operating over about 100 Amps. Reduced performance occurs in part because the extended small diameter tip portion 150 of the nozzle 110 (i.e., the part of the nozzle 110 that extends axially towards the shield exit portion 135) tends to overheat. Overheating can degrade the quality of a cut and cause premature failure of the nozzle 110. Another limitation is the relatively long axial length 155 of the shield exit orifice that is required to establish an axially directed column of shielding gas. The longer axial length 155 generally involves relatively large distances between the nozzle 110 and the workpiece (not shown).
FIG. 1B is a sectional view along the plane 1-1′ of the torch 100 of FIG. 1A. FIG. 1B illustrates the origin of the coaxial columns of gas flow. The plasma jet is ejected from the torch 100 via the plasma exit portion 130 of the nozzle 110. The column of gas is generated in the space 160 between an outer surface 165 of the nozzle 110 and an inner surface 170 of the shield exit portion 135 of the shield 120. The difference in diameter between the plasma exit portion 130 and the space 160 allows the secondary gas to enclose the plasma jet.
A particular problem encountered when using a shield gas to improve arc stability occurs when the shield gas impinges on or interferes with the plasma jet itself. In one known configuration, a plasma arc torch includes an electrode and a nozzle mounted in spaced relationship with a shield to form one or more passageways for fluids (e.g., shield gas) to pass through a space disposed between the shield and the nozzle. Plasma gas flow passes through the torch along the torch's longitudinal axis (e.g., about the electrode, through the nozzle, and out through the nozzle exit orifice). The shield gas or other fluid passes through the one or more passageways to cool the nozzle and impinges the ionized plasma gas flow at a 90 degree angle as the plasma gas flow passes through the nozzle exit orifice. As a result of the impingement, the ionized plasma gas flow can be disrupted (e.g., generating instabilities in the plasma gas flow), which may lead to degraded workpiece processing.